The present invention relates to a method for producing asymmetric radial polymers. More particularly, this invention relates to a one reactor method for producing asymmetric radial polymers of conjugated dienes and/or vinyl aromatic hydrocarbons and also other anionically polymerizable monomers such as acrylic monomers.
Heretofore, several methods have been proposed for preparing asymmetric radial polymers. As is well known in the prior art, radial polymers comprise three or more arms extending outwardly from a nucleus. The asymmetric radial polymers contain arms of at least two different polymers, which polymers may vary as to chemical composition, structure and/or molecular weight. A principal difference in the methods frequently used to prepare asymmetric radial polymers resides in the selection of a coupling agent which forms the nucleus of the radial polymer. Multifunctional coupling agents such as silicon tetrachloride have been used to form asymmetric radial polymers having three or four arms. Star-shaped radial polymers having many more arms have been formed using a poly alkenyl aromatic compound, such as divinyl benzene, as the coupling agent as described in Canadian Patent 716,645.
Before 1988, such asymmetric radial polymers were made by forming a blend of the different polymeric arms in the desired ratio and then adding the coupling agent to couple the arms to the coupling agent. Those methods resulted in a product having, on average, the desired number of each kind of arm in the asymmetric polymer. The problem associated with producing asymmetric polymers in that manner is that the product obtained is actually a statistical distribution of all possible products. Unfortunately, this means that a certain percentage of the polymer produced would constitute a symmetric radial polymer containing polymer arms of only one type that could adversely affect the characteristics of the polymer.
In 1988, one of the present applicants filed a patent application which resulted in U.S. Pat. No. 4,879,349. Therein is disclosed a two-reactor process for producing asymmetric radial polymers which avoids the problem of the production of the unwanted homopolydiene polymer and other undesirable species. The process involves separately polymerizing the monomers to create separately the two different types of arms (it was not thought possible to polymerize the arms in the same reactor and still achieve a product which is not a statistical blend). Then one of the polymeric arms is coupled to the coupling agent and when that coupling reaction is complete, the second set of polymer arms is coupled to the coupling agent. This maximizes production of the desired species of asymmetric radial block copolymer. The two-reactor process described above is very advantageous and produces polymers which have very good properties and are useful in adhesive compositions and for a wide variety of other uses. However, the two-reactor process does have the disadvantage that it is more expensive to carry out. It requires additional capital expenditure for a second reactor and additional time to produce the final polymer.
Therefore, it would be very advantageous to have a method which is capable of producing in one reactor an asymmetric radial polymer maximizing the amount of the desired species of polymer which is produced. The present invention provides such a process and produces such a polymer.